1. Field of the Invention
The present invention relates to a color thermal printer and printing method. More particularly, the present invention relates to a color thermal printer and printing method capable of overcoming considerable smallness or lowness in the extent of optical fixation.
2. Description Related to the Prior Art
A color thermal printer of a direct printing type is used with a color thermosensitive recording material or recording sheet, in which thermosensitive coloring layers of cyan, magenta, and yellow are formed on a support. To print a full-color image, a thermal head is pressed against the recording sheet and applies heat to it, so as to record a yellow image to the yellow coloring layer one line after another at first. The yellow coloring layer has the highest heat sensitivity of the three layers. There is a yellow fixing ultraviolet lamp for emitting near ultraviolet rays peaking at a wavelength of 420 nm. Once the yellow coloring layer is heated, the yellow fixing lamp applies the near ultraviolet rays to it to fix it optically before heat energy for the magenta coloring layer next to be colored is applied. This is to destroy the further coloring ability of the yellow coloring layer to prevent it from being colored beyond desired density.
After yellow fixation, the thermal head records a magenta image to the magenta coloring layer one line after another. There is a magenta fixing ultraviolet lamp for emitting ultraviolet rays peaking at a wavelength of 365 nm. The magenta fixing lamp applies ultraviolet rays to fix it optically. Then the thermal head records a cyan image to the cyan coloring layer one line after another. The three-color frame-sequential recording is finished, to print the full-color image to the recording sheet.
During the printing operation, the recording sheet is conveyed past the thermal head continuously or intermittently one line after another. There are two types of a sheet conveying mechanisms: a back-and-forth moving type in which a pair of conveyor rollers make back-and-forth movement of the recording sheet three times; and a rotational type in which a platen drum of large size supports the recording sheet and makes three rotations.
When a ray emitting intensity of the yellow fixing lamp is too low, the yellow image is only fixed to a considerably small or low extent. Inevitably the remainder of the yellow coloring component after the yellow recording is colored in the recording of the magenta image. If the ray emitting intensity of the yellow fixing lamp is too high, a slight part of the magenta coloring component is decomposed optically, so that the magenta image only can be printed at the lowered density. It is therefore suggested to use a sensor for measuring the near ultraviolet rays from the yellow fixing lamp. Irradiance of the near ultraviolet rays immediately after the actuation is measured by the sensor. According to this, a driving condition of the yellow fixing lamp is determined to maintain the ray emitting intensity in a desired level.
When the considerable smallness or lowness in the extent of fixation occurs for the magenta image, the remainder of the magenta coloring component is colored in the cyan recording. However the cyan coloring layer does not have optical fixability, and is never influenced by overfixation of the magenta image. Consequently the magenta fixing lamp is fully driven without decreasing adjustment in the fixation of the magenta image.
A normal ultraviolet lamp has a phenomenon in which mercury is precipitated in the ultraviolet lamp, and deposited on the inside of its tube according to the number of times of the use of the ultraviolet lamp. It has been found that the deposits of the mercury cause the ultraviolet lamp to emit rays only at a small amount even with an equal power at which the lamp is driven.
In the conventional color thermal printer, the fixing condition is determined in such a manner that an exposure amount is optimized by fully driving the magenta fixing lamp as the ultraviolet lamp. The magenta fixing lamp having deposits of mercury is involved with the considerable smallness or lowness in the extent of fixation of the magenta image when the ray emitting intensity is minimal. The same problem occurs to an alternative recording sheet in which a magenta coloring layer is the deepest and a yellow coloring layer is the second deepest. The yellow image is no more fixed than to a considerable small extent.